Steps:

(1) Download VirtualBox 4.2.20 and install it on Macbook;(2) Download Ubuntu 12.04 AMD 64 Desktop from its official website;(3) Create a virtual machine in VirtualBox (Do note that you need to specify the "Type" as "Linux" and the "Version" as "Ubuntu (64 bit)") with a disk space more than 10GB;(4) Mount the downloaded Ubuntu image to the CDROM of the created virtual machine;(5) Start the virtual machine and install Ubuntu on the machine;(6) After the installation is done, start virtual machine and login the Ubuntu just installed;(7) The default screen resolution is 1024*768, which is surely too small, to fix which we click the menu "Devices -> Install Guest Additions" of VirtualBox VM; in the Ubuntu, you will find some pop-up window asking you whether to install the new package; just follow the instructions to install the guest additions; please do not install the the guest additions using apt-get in Ubuntu, which may crash the whole Ubuntu somehow;(8) Restart the virtual machine, and now the screen size can be automatically changed to fit the window size of the virtual machine.(9) If you need Ubuntu to display emojis correctly, you have to install some new fonts: download Symbola.ttf from http://users.teilar.gr/~g1951d/ in the Ubuntu virtual machine, and then double click the ttf file and click install to add the font to Ubuntu.